Radioisotopes have been used for a number of years to excite phosphors into luminescing to produce visible light. One example of such a use is the common watch or alarm clock having a dial that is visible in the dark. The clock hands and numeration points around the dial are coated with a paint containing a radioisotope which excites a phosphor to produce visible light. The phosphors will continue to produce light visible in darkness as long as the radioactive decay or activity of the radioisotope is sufficient to excite the phosphors.
It has long been recognized that it would be desirable to produce much brighter light sources using radioluminescence, if such could be done safely and economically. Such lights could be used, for example, as airfield runway edge and threshold markers. Conventional airfield lights consume large amounts of electrical power and are not practical in remote areas. A radioluminscent light would have its own inherent power source that could last for many years with virtually no maintenance, except perhaps for periodic cleaning of the light surface. Such concepts could also be employed to produce escape and exit signs in buildings which must be viewable when electrical power is interrupted. The present use of a back-up battery system for supplying power to such lights could be eliminated by employing a safe and sufficiently bright radioluminescent light source.
Further, it would be desirable to use radioluminescent sources that produce electromagnetic radiation in the infrared region of the light spectrum. This would provide a simulated thermal source which might have application in military and counter-intelligence operations.
Prior art research and development efforts regarding high intensity radioluminescent light sources have produced less than desirable brightness. Much of the recent research and development has focused upon light sources which employ the radioactive decay of the hydrogen isotope tritium. One such light source, produced by Oak Ridge National Laboratories, employs inorganic phosphors which are activated by the beta particles (electrons) produced during decay of tritium gas. The light source employs an elongated, closed tube approximately 10 inches long and 1 inch wide. The volume of the tube cavity is 14.4 cm.sup.3. A phosphor is coated about the inner tube surface, and the tube loaded with tritium gas. The volume of tritium in the tube produces a radioactive decay rate of 50 Ci. Such a light source is reportedly capable of producing light at the rate of 1.0 ft-Lambert, which corresponds to 2.0 micro-Lamberts per milliCurie. Under optimum conditions in near complete darkness, such a light source would be expected to be viewable by the unaided human eye from a distance of approximately 1 to 1.5 miles.
Although this light source is quite functional, it does present a theoretical hazard due to the gaseous tritium enclosed within the system. Were such a light fractured, the gaseous tritium would undoubtedly escape into the atmosphere. Additionally, many radioluminescent paints that utilize tritium decay leak tritium to the environment, which poses a hazard. Furthermore, it would be desirable to increase the amount of luminosity produced for a given amount of radioactive decay to both maximize efficiency and minimize the amount of radioactive decay necessary to produce a given intensity light for safety reasons.